How One of California’s Largest Reservoirs Permanently Lost Room for 36 Billion Gallons of Water

California got a particularly rainy winter, but state officials have uncovered another reason why Lake Oroville overflowed with water this spring. The massive reservoir, the state’s second largest behind Lake Shasta, has slowly but surely shrunk.

New research from the California Department of Water Resources shows that the lake, used for millions of Californians’ drinking water and irrigating hundreds of thousands of acres of farmland, isn’t quite the size it once was. The agency released its findings June 26, writing that in the 56 years since Lake Oroville was filled, rock and silt settling on the reservoir floor have cut its capacity by almost 113,000 acre-feet, or more than 36 billion gallons.